Nearest point to a z(x,y) function
You can get an intersection using raycast BVH tree node. I have used your node set up and added an icosphere and an empty and have always execute option ticked in AN as shown. Just grab the empty and move it around.

Make sure the always executing box is ticked so it updates when moving the empty.

And if you want to find the point on the mesh nearest to the intersection just add these three nodes.

If you move the empty below the mesh it will not work as the Ray Cast BVH Tree is just projecting downwards in -z direction from the empty. To solve that, tick the&quot;Start in infinity&quot; box in the Ray Cast BVH Tree node, it will then project in the  opposite direction aswell.